Fluidity and the Pitch Drop Experiment

It’s probably not the most important scientific research project to come out of Queensland, but it may well be the most famous.  In 2005 the University of Queensland Physics Department’s ‘pitch drop experiment’ won the Ig Nobel Prize.  According to the 2002 Guinness Book of Records, it is the oldest continuously running scientific experiment in the world.  It has its own YouTube site.

What is the difference between a solid and a liquid?  For most materials, the answer is simple – water is a liquid, ice is a solid – but for some materials, the answer is less straightforward.  Which category does glass fit into, for instance?  It is often thought that it flows very slowly, so that gravity gradually distorts the shape of stained glass windows so that they are discernibly thicker at the bottom.  This seems to be disputed: hand-blown medieval glass panels are distorted, but it may be that cathedral builders very sensibly placed the heaviest, thickest parts of the glass on the lower side of a panel as this would be the more stable arrangement.

In 1927, Professor Thomas Parnell, the first professor of physics at the University of Queensland, set up a demonstration for his students of the way that something apparently solid, pitch, is capable of behaving like a liquid, though a very viscous liquid.  He heated a sample of pitch until it was liquid and poured it into a large glass funnel with a sealed stem.  He waited three years for it to cool and consolidate back to its solid state.  Then, in 1930, he cut the glass stem of the funnel and the pitch began very slowly to flow.